It's not perfect, certainly. But it's also not easy to get mitres right whether it's coving, skirting, or floors, which are rarely either level or square at the corners. Houses, even modern ones, are all over the place when it comes to how square, flat, level, etc anything is, so often you have to compromise. I've been doing some coving- when the wall corner isn't square- and they never are- it's a hell of a job getting the corners to fit perfectly.
Is this just in the corners of the room, and/or just round the walls? If so, most of it will be under furniture and you may not notice it. I always find that when I'm doing a job I get more and more perfectionist about it, spending hours getting things just right, just square, whatever. But I know that a week after it's done, I will stop noticing that level of detail.
When we laid an engineered floor, which we've only done once, we avoided mitres, laying it all in straight parallel planks. Then we could deal with non-square walls by just tapering the planks at the edges. If the walls weren't square in my house but I wanted mitres, I'd have laid the floor square working from one wall (the most visible one) in a straight line, and taken up the slack in the same way, by tapering the edges. But then I'd have had to accept that the outer edges on the other three walls might taper quite a lot, which might look bad around doors etc.. Something has to give. I may be wrong, but maybe your person has decided to let the mitres take the strain instead so that the edges don't taper?